Calibration vial and technique for calibrating a fiber optic oxygen sensing needle

ABSTRACT

A calibration tool and method of using the tool to calibrate a fiber optic needle oxygen sensor. The tool includes at least a vial sealingly covered by a septa and containing a supply of particulate oxygen getter within the chamber of the vial. The vial has an open top and is constructed from an oxygen impermeable material. The septa is resealing, needle-penetrable and oxygen impermeable. The supply of particulate oxygen getter is retained within an oxygen permeable sachet.

BACKGROUND

Photoluminescent sensors or probes are a widely employed method of measuring analyte concentration, typically oxygen, within an enclosed space such as a package, container or living tissue. Briefly, analyte concentration within a package or container can be measured by inserting an analyte sensitive photoluminescent probe within the package, container or tissue, allowing the probe to equilibrate within the package, container or tissue, exciting the probe with radiant energy, and measuring the extent to which radiant energy emitted by the excited probe is quenched by the presence of the target analyte. Exemplary optical sensors are described in WO 92/19150 and CA 2,600.563. Such optical sensors are available from a number of suppliers, including Presens Precision Sensing, GmbH of Regensburg, Germany, Oxysense of Dallas, Tex., United States, and Luxcel Biosciences, Ltd of Cork, Ireland.

Analytical instruments used to read such photoluminescent probes are commonly programmed with a calibration mode that permits calibration of the instrument by having the instrument read probes that have been exposed to mediums having known concentrations of the target analyte (e.g., setting the instrument to calibration mode, reading a probe that has been placed within a container that is flushed with certified tank gas containing 0% analyte, and then reading a probe that has been placed within a container that is flushed with certified tank gas containing a known concentration(s) of analyte such as 100% analyte).

While effective for accurately calibrating optical sensors, this calibration method is time consuming and expensive.

Accordingly, a substantial need exists for a low cost system and method for accurately and reliably calibrating instruments used to read photoluminescent sensors or probes.

SUMMARY OF THE INVENTION

A first aspect of the invention is a calibration tool for use in obtaining a zero calibration value for a fiber optic needle oxygen sensor. The calibration tool includes a vial sealingly covered by a septa and containing a supply of particulate oxygen getter within the chamber of the vial. The vial has an open top and is constructed from an oxygen impermeable material. The septa is resealing, penetrable by a fiber optic needle oxygen sensor, and oxygen impermeable. The supply of particulate oxygen getter is retained within an oxygen permeable sachet.

A second aspect of the invention is a method of calibrating a fiber optic needle oxygen sensor having an oxygen sensitive photoluminescent probe proximate the tip of the needle. The calibration method includes the steps of (a) setting the sensor to calibration mode, (b) obtaining a span calibration value by (i) placing the photoluminescent probe on the sensor in fluid communication with a fluid having a known non-zero partial pressure of oxygen, (ii) taking an oxygen concentration reading with the photoluminescent probe in fluid communication with a fluid having a known non-zero partial pressure of oxygen, and (iii) correlating the oxygen concentration reading with the known non-zero oxygen partial pressure, and (c) obtaining a zero calibration value by (i) placing the photoluminescent probe on the sensor in fluid communication with the chamber of a calibration tool in accordance with the first aspect of the invention by inserting the needle through the septa on the calibration tool, (ii) taking an oxygen concentration reading with the photoluminescent probe in fluid communication with the chamber of the calibration tool, and (iii) correlating the oxygen concentration reading with the known near zero oxygen partial pressure in the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded side view of one embodiment of the invention depicted in combination with a fiber optic needle oxygen sensor.

FIG. 2 is an enlarged cross-sectional side view of the cap component of the invention depicted in FIG. 1.

FIG. 3 is a grossly-enlarged cross-sectional side view of a portion of the sachet component of the invention depicted in FIG. 1 showing the particulate oxygen getter contained within the sachet.

FIG. 4 is a grossly-enlarged cross-sectional side view of the tip of the fiber optic needle oxygen sensor depicted in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Definitions

As used herein, including the claims, the phrase “oxygen impermeable” means a material that when formed into a 1 mil (25.4 μm) film has an oxygen transmission rate of less than 10 cm³/m² day when measured in accordance with ASTM F 1927.

As used herein, including the claims, the phrase “oxygen permeable” means a material that when formed into a 1 mil film has an oxygen transmission rate of greater than 1,000 cm³/m² day when measured in accordance with ASTM D 3985.

As used herein, including the claims, the phrase “oxygen barrier” means a film, including coated, metalized and multiple layer films, that are impervious to oxygen (such as a layer of metal) or have an oxygen transmission rate of less than 0.1 cm³/m² day when measured in accordance with ASTM F 1927.

As used herein, including the claims, the phrase “near zero”, when used to describe a concentration of oxygen in a sample, means less than 0.01% oxygen.

As used herein, including the claims, the phrase “oxygen sensitivity” or “sensitivity to oxygen” means sensitivity measured by luminescence quenching.

As used herein, including the claims, the phrase “thin film” means a film having a thickness of less than 10 μm.

As used herein, including the claims, the phrase “small container” means a container will a fillable volume of less than 20 ml.

Nomenclature

-   10 Calibration Tool -   20 Vial -   20 a Open Top of Vial -   20 b Bottom of Vial -   29 Vial Retention Chamber -   30 Cap -   31 Septa -   31 ₁ Resealable Layer of Septa (Elastomer) -   31 ₂ Oxygen Barrier Layer of Septa (Metal Foil) -   40 Sachet -   41 Particulate Oxygen Getter -   50 Confirmation Photoluminescent Oxygen Probe -   S Fiber Optic Needle Oxygen Sensor -   S₁ Tip of Fiber Optic Needle Oxygen Sensor -   S₂ Side Port In Fiber Optic Needle Oxygen Sensor -   F Fiber Optic Filament -   P Photoluminescent Oxygen Probe     Construction

FIG. 1 depicts a calibration tool 10 constructed in accordance with the invention. The calibration tool 10 includes a vial 20, a cap 30 and a sachet 40 containing a particulate oxygen getter 41.

The vial 20 or other suitable container has an open top 20 a and a closed bottom 20 b. The vial 20 preferably has a fillable volume of less than 50 ml, with a preference for small containers having a fillable volume of between 2 and 20 ml.

The vial 20 can be constructed from substantially any oxygen impermeable material possessing the necessary structural integrity. The vial 20 is preferably constructed from an oxygen barrier material for reducing the rate at which oxygen permeates through the walls of the vial 20 and into the chamber 29 defined by the vial 20. Suitable materials include specifically, but not exclusively, glass and certain plastics. Glass is preferred.

Referring to FIGS. 1 and 2, the open top 20 a of the vial 20 is sealingly covered by a cap 30 with a septa 31 or other suitable sealing device. In addition to sealing off the chamber 29 of the vial 20 from the surrounding environment, the cap 30, and in particular the septa 31 on the cap 30, needs to be impermeable to oxygen—at least prior to being penetrated by the tip S₁ of a fiber optic needle oxygen sensor S, and reseal after being penetrated by the tip S₁ of a fiber optic needle oxygen sensor S. Use of a cap 30 which is not impermeable to oxygen can significantly reduce the useful lifespan of the calibration tool 10 by rapidly utilizing the oxygen getter retained within the chamber 29 even prior to use, and in egregious cases can transmit oxygen at a rate faster than the oxygen getter 41 retained within the chamber 29 can scavenge the oxygen, causing the oxygen concentration within the chamber 29 to almost instantaneously rise well above zero. Use of a cap 30 which cannot reseal after being penetrated by the tip S₁ of a fiber optic needle oxygen sensor S effectively renders the calibration tool 10 an expensive single use disposable item as oxygen from the surrounding environment will readily pass through the hole left in the cap 30 after a single fiber optic needle oxygen sensor S is calibrated with the calibration tool 10.

Referring to FIG. 2, one type of cap 30 capable of providing both the desired resealing and oxygen impermeable characteristics is a cap 30 with a septa 31 comprised of a layer of resealing elastomer 31 ₁, such as synthetic silicone rubber, with a layer of an oxygen impermeable material 31 ₂, such as a metal foil, laminated to the underside thereof. Caps 30 with such dual functional septa 31 are available from a number of sources, including Sigma-Aldrich and Fisher Scientific. The cap 30 can be selected from any style cap including specifically, but not exclusively screw and crimp style caps, with crimp style caps generally preferred.

Referring to FIGS. 1 and 3, particulate oxygen getter 41 is provided within the chamber 29 of the vial 20 for consuming any oxygen that reaches the chamber 29 through the vial 20 and/or the cap 30 in order to maintain the concentration of oxygen within the chamber 29 at near zero.

The oxygen getter 41 can be selected from any known oxygen getter, including specifically but not exclusively activated carbon, silica gel, activated alumina, molecular sieve, metals such as iron, magnesium, zinc and titanium, and various inorganic salts of such metals.

As is known to those of routine skill in the art, a particulate hygroscopic material (not shown) is frequently employed in combination with a particulate oxygen getter 41 as many oxygen getters 41 require water in order to chemically react with and scavenge the oxygen, and water absorbed by a hygroscopic material can provide the necessary supply of water.

The oxygen getter 41 must have an oxygen consumption rate that exceeds the permeation rate of oxygen into the chamber 29 from the surrounding air. Generally, the oxygen getter 41 preferably consumes oxygen at a rate that is only slightly greater than the permeation rate of oxygen into the chamber 29, with a generally acceptable range of between two to ten times the permeation rate of oxygen into the chamber 29. One of the factors that impacts the speed with which the oxygen getter 41 can scavenge oxygen within the chamber 29 is the surface area of the oxygen getter 41. In order to provide a surface area effective for scavenging oxygen reaching the chamber 29 at a rate that is at least as fast as the rate at which oxygen permeates into the chamber 29 from the surrounding air, the oxygen getter 41 is preferably provided in particulate or powdered form with an average particle size of less than 10 mil, preferably 1-5 mil and most preferably 1-3 mil. However, when provided in this form I discovered that the particulates have a tendency to diffuse into and coat the probe P and fiber optic filament F on fiber optic needle oxygen sensors S inserted into the chamber 29 containing these particulates, with a concomitant reduction in the accuracy and sensitivity of the contaminated sensor S. Referring to FIGS. 1 and 3, I discovered that this drawback could be eliminated, without significantly impacting the oxygen scavenging performance of the particulate oxygen getter 41, by retaining the oxygen getter 41 in an oxygen permeable sachet 40. Such sachets 40 filled with particulate oxygen getter material 41 are commercially available from a number of suppliers, typically sold for purposes of protecting packaged foods, including Multisorb Technologies under the designation FreshPax®.

The outside of the vial 20 can be imprinted with indicia (not shown) identifying the vial 20 as a zero-calibration tool 10 (e.g., “0% O₂”, “Zero O₂”, etc.).

One of the factors impacting the lifespan of the calibration tool 10 is the ratio of oxygen getter 41 to chamber size 29. A lifespan of at least one year from sealing of the chamber 29 or 100 uses, whichever occurs first, is generally desired. In order to reach this desired lifespan, a weight/volume ratio of oxygen getter 41 to sealed chamber 29 of 1:5 to 1:20 mg/cm³ is generally effective, with a weight/volume ratio of 1:10 to 1:20 mg/cm³ generally preferred.

The calibration tool 10 can optionally be equipped with a confirmation probe 50 within the chamber 29 for allowing the oxygen concentration within the chamber 29 to be checked by interrogating the confirmation probe 50 with a photoluminescent reader (not shown).

Vials 20 equipped with a confirmation probe 50 within the chamber 29 need to be transparent or translucent at least at the specific wavelengths at which the confirmation probe 50 absorbs and emits energy.

The oxygen sensitive confirmation probe 50 can be constructed in accordance with standard convention by embedding an oxygen-sensitive photoluminescent dye (not shown) within an oxygen-permeable carrier matrix (not shown).

The oxygen-sensitive photoluminescent dye (not shown) may be selected from any of the well-known oxygen sensitive photoluminescent dyes used in the construction of oxygen sensitive photoluminescent probes. A nonexhaustive list of such oxygen sensitive photoluminescent dyes (not shown) includes specifically, but not exclusively, ruthenium(II)-bipyridyl and ruthenium(II)-diphenylphenanothroline complexes, porphyrin-ketones such as platinum(II)-octaethylporphine-ketone, platinum(II)-porphyrin such as platinum(II)-tetrakis(pentafluorophenyl)porphine, palladium(II)-porphyrin such as palladium(II)-tetrakis(pentafluorophenyl)porphine, phosphorescent metallocomplexes of tetrabenzoporphyrins, chlorins, azaporphyrins, and long-decay luminescent complexes of iridium(III) or osmium(II).

Compositions suitable for use as the carrier matrix (not shown) are oxygen-permeable compositions, preferably highly oxygen permeable compositions. One of routine skill in the art is capable of selecting such oxygen-permeable carrier compositions (not shown). A nonexhaustive list of polymers suitable for use as the carrier matrix (not shown) includes specifically, but not exclusively, silicone, polystryrene, polycarbonate, polysulfone, and some other polymers and co-polymers.

Use

The calibration tool 10 can be used to quickly and easily calibrate a fiber optic needle oxygen sensor S having an onboard oxygen sensitive photoluminescent probe P and programmed with a calibration mode. Calibration of the sensor S simply involves the steps of (1) setting the sensor S to calibration mode, (2) obtaining a span calibration value by (i) placing the photoluminescent probe P on the sensor S in fluid communication with a fluid having a known non-zero partial pressure of oxygen (typically atmospheric air known to have a stable concentration of 20.95% O₂), (ii) taking an oxygen concentration reading with the photoluminescent probe P onboard the sensor S in fluid communication with such fluid, and (iii) correlating the oxygen concentration reading with the known non-zero oxygen partial pressure, and (3) obtaining a zero calibration value by (i) placing the photoluminescent probe P onboard the sensor S in fluid communication with the chamber 29 of a calibration tool 10 by inserting the tip of the needle S₁ through the septa 31 covering the chamber 29, (ii) taking an oxygen concentration reading with the photoluminescent probe P in fluid communication with the contents of the chamber 29, and (iii) correlating the oxygen concentration reading with the known near zero oxygen partial pressure in the chamber 29.

Correlation of each oxygen concentration reading to the proper calibration point (i.e., span calibration reading or zero calibration reading) can be accomplished in various ways. One technique is to take the oxygen concentration readings in a predetermined sequence previously input into the optical oxygen sensor S. A second technique is to provide the optical oxygen sensor S with additional data each time a reading is taken effective for indicating which calibration point has been or will be taken. A third technique is to provide the optical oxygen sensor S with additional data each time a reading is taken effective for indicating the oxygen concentration to which probe P was exposed at the time of the reading (e.g., user input of 20.95% O₂ after reading air and 0% O₂ after reading the chamber 29 of the calibration tool 10.). 

I claim:
 1. A calibration tool for use in obtaining a zero calibration value for a fiber optic needle oxygen sensor, consisting essentially of: (a) an open top vial constructed from an oxygen impermeable material and defining a chamber, (b) a resealing, needle-penetrable, oxygen impermeable septa sealingly engaged over the open top of the vial, and (c) an oxygen permeable sachet containing a mass of particulate oxygen getter retained within the sealed chamber.
 2. The calibration tool of claim 1, wherein the vial is a small container.
 3. The calibration tool of claim 1, wherein the oxygen getter is iron powder having an average particle size of 1 to 5 mil.
 4. The calibration tool of claim 2, wherein the weight/volume ratio of iron/sealed chamber is 1:10 to 1:20 mg/cm³.
 5. A method of calibrating a fiber optic needle oxygen sensor having an oxygen sensitive photoluminescent probe proximate the tip of the needle, comprising the steps of: (a) setting the sensor to calibration mode, (b) obtaining a span calibration value by (i) placing the photoluminescent probe of the sensor in fluid communication with a fluid having a known non-zero partial pressure of oxygen, (ii) taking an oxygen concentration reading with the photoluminescent probe in fluid communication with a fluid having a known non-zero partial pressure of oxygen, and (iii) correlating the oxygen concentration reading with the known non-zero oxygen partial pressure, and (c) obtaining a zero calibration value by (i) placing the photoluminescent probe of the sensor in fluid communication with the chamber of the calibration tool of claim 2 by inserting the needle through the septa on the calibration tool, (ii) taking an oxygen concentration reading with the photoluminescent probe in fluid communication with the chamber of the calibration tool, and (iii) correlating the oxygen concentration reading with the known near zero oxygen partial pressure in the chamber.
 6. The method of claim 5 wherein the span calibration value and the zero calibration value are taken in a predetermined sequence.
 7. The method of claim 5 further comprising the step of providing the oxygen sensor with data indicating whether a first reading was a span calibration value or a zero calibration value.
 8. The method of claim 5 wherein the span calibration value is obtained by placing the photoluminescent probe in fluid communication with the atmosphere. 